Although scarce in nature, pyrazole, its derivatives and physiologically compatible salts have found uses in many areas, such as pharmaceuticals, agricultural chemicals and hair dyes. Since the discovery of the high potential of these pyrazole derivatives, many publications have been devoted to the synthesis of pyrazoles and related compounds.
In particular, in the oxidative hair dyeing field, 1-substituted-4,5-diaminopyrazole of general formula (II) and salts thereof have shown to be interesting primary intermediates, providing a wide pallet of colour when used with various couplers in the presence of an oxidative dyeing medium.

U.S. Pat. No. 6,452,019 discloses an improved process for the preparation of 4,5-diamino-1-(2′-hydroxyethyl)pyrazole and acid addition salts thereof such as the addition salt from sulfuric acid. The process comprises a combination of steps beginning with an alkyl(alkoxymethylene)cyanoacetate and 2-hydroxyethylhydrazine and the formation of intermediate compounds 5-amino-4-alkoxycarbonyl-1-(2′-hydroxyethyl)pyrazole, 5-amino-4-carboxyl-1-(2′-hydroxyethyl)pyrazole, 5-amino-1-(2′-hydroxyethyl)pyrazole, 5-amino-1-(2′-hydroxyethyl)-4-nitrosopyrazole.
U.S. Pat. No. 5,663,366 describes a process for making 4,5-diaminopyrazole derivative compounds of the formula (D):

wherein R1 and R2 are independently selected from the group consisting of hydrogen, alkyl radicals having one to six carbon atoms and hydroxyalkyl radicals having two to four carbon atoms, provided that R2 may not be tertiary butyl. In said process a) the 3,5-dibromo-4-nitropyrazole (A) is first alkylated in the 1-position by converting with C1- to C6-alkyl halides, C2- to C4-hydroxyalkyl halides or benzyl halides in dimethylformamide (DMF) (method I) or by converting with C1- to C6-alkyl sulfate, C2- to C4-hydroxyalkyl sulfate or benzyl sulfate and caustic solution (method II); b) in a subsequent step, the N-substituted 3,5-dibromo-4-nitropyrazoles of general formula (B) are heated in an aqueous, alcoholic or aqueous-alcoholic solution of C1- to C6-alkyl amine, C2- to C4-hydroxyalkyl amine or benzyl amine or in the corresponding amine itself, as solvent, at a temperature of 60° C. to 80° C.; and c) the compounds of general formula (C) are then hydrogenated using a palladium-on-activated-carbon catalyst with a palladium content of 10 percent by weight to produce compounds of general formula (D).
Hans Höhn describes a synthesis of pyrazole derivatives in the journal “Zeitschrift für Chemie”, 10(10), 386-8; 1970.

The present invention describes a new synthetic route to reach the 1-substituted-4,5-diaminopyrazole of general formula (II) via the key intermediate of the general formula (I).
In particular, the invention provides a sequential one-pot synthesis; with reagents added to a reactor one at a time and without work-up in between. Thus, the major advantage of the process provided by the present invention is to prevent the “workup”; i.e. the several manipulations usually required at the end of a chemical reaction, in order to isolate and purify the intermediates. In addition we can avoid contact with toxic intermediates such as hydrazine. Hydrazine itself and some alkyl- and phenyl-substituted derivatives are officially classified as carcinogens Care IB (former class carc cat 2 under DSD) in Annex VI of the CLP regulation 1272/2008/EC. As a final step any un-reacted hydrazine can be degraded by an excess of the nitroso source present in the reaction mixture into nitrogen gas.